Finding Epicenters Lab#______

Regents Earth Science
Unit 3: Dynamic Crust
Finding Epicenters
Introduction: Earthquakes occur when there is movement along a fault. The friction
between rock masses rubbing against one another generates shock waves which travel
through the Earth. These shock waves (seismic waves) created by the earthquake are
radiated in every direction from the focus, the point within the Earth where the actual
movement takes place.
An earthquake occurs every 30 seconds, day after day. Most of these are so weak
they would go unnoticed without the use of sensitive modern instruments called
While seismologists use many stations, in this lab you will use seismograms from three
distant stations to locate the epicenter, the point on the Earth’s surface directly above the
Objective: You will learn to interpret a seismogram and, using differences in seismic waves,
locate the epicenter of an earthquake.
Procedure A:
Use the three seismograms provided on page 4, and the Earthquake P-wave and S-wave
Travel Time chart in the Earth Science Reference Tables, to calculate the following for each
city: (Fill in all of the required information on the Report Sheet on page 5).
1. The arrival times for P and S-waves.
2. The difference in the arrival time between P and S-waves.
3. The distance (in km) of the epicenter from each city.
4. The length of time it took for the P-wave to travel from the epicenter to each city.
5. Since you now know when the P-wave arrived at a city and how long it had to travel,
calculate the time at which the P-wave started (Origin Time).
Procedure B:
1. a. Find the Epicenter Location Map on page 6. To locate the epicenter on the map,
for each city construct a circle whose radius is equal to the distance from the city to
the epicenter.
b. Use the scale of distance of your map to set the drawing compass at the correct
2. Mark and label the epicenter on the map where all three circles intersect.
3. Answer the Analysis and Conclusion Questions.
Analysis and Conclusion Questions:
1. How do P-waves and S-waves differ?
2. What was the approximate location of the epicenter of this earthquake?
The cross section of Earth below shows a P-wave moving away from an earthquake epicenter to seismic
station A.
3. No S-waves arrive directly at seismic station A because
(1) some parts of the core are liquid
(2) S-waves travel too slowly
(3) the distance to seismic station A is too great
(4) seismic station A is located on glacial ice
Base your answers to questions 4 and 5 on the diagrams below. Diagram 1 represents a cross section of Earth and its
interior layers. The asterisk (∗ ) shows the location of an earthquake epicenter. Letters A through D are seismic
stations on Earth’s surface.
Diagram 2 shows four seismograms labeled I, II, III, and IV, which were recorded at seismic stations A, B, C, and D
during the same time interval.
4. Which list correctly matches the seismograms with the seismic stations where they were recorded?
seismogram I – station A
seismogram II – station B
seismogram III – station C
seismogram IV – station D
seismogram I – station C
seismogram II – station B
seismogram III – station D
seismogram IV – station A
seismogram I – station B
seismogram II – station D
seismogram III – station A
seismogram IV – station C
seismogram I – station A
seismogram II – station D
seismogram III – station B
seismogram IV – station C
5. Station D is 8000 kilometers from the earthquake epicenter. How long did it take for the first P-wave to travel from the
epicenter to station D?
(1) 9 minutes 20 seconds
(2) 11 minutes 20 seconds
(3) 20 minutes 40 seconds
(4) 4 minutes 20 seconds
6. The distance from Albany, New York, to the epicenter of an earthquake is 5600 km. Approximately how much longer did
it take for the S-wave to arrive at Albany than the P-wave?
(1) 4 minutes and 20 seconds
(2) 7 minutes and 10 seconds
(3) 9 minutes and 0 seconds
(4) 16 minutes and 10 seconds
Base your answers to questions 7 through 10 on the map below, which shows a portion of southwestern
United States. On January 17, 1994, an earthquake occurred with an epicenter at Northridge, California.
7. State the latitude and longitude of Northridge, California. Include the correct units and compass directions in your answer.
8. Explain why earthquakes are common in this region of California.
9. Of the cities shown on the map, explain why Oakland was the last city to receive P-waves from this earthquake.
10. List two actions that a homeowner could take to prepare the home or family for the next earthquake.
Arrival (clock times)
Difference in
Arrival Time
(min. and sec.)
Distance to P-wave Travel
(min. and sec.)
Time of Origin
(hr., min. and